The present invention relates generally to optical amplifiers used in fiber optics for telecommunications. More particularly, the invention relates to an optical fiber amplifier and a method and apparatus for spectral equalization of multi-channel amplified output at multiple gain values.
In optical communication systems using wavelength division multiplexing (WDM) several streams of information, each at different wavelength channel are being transmitted. This information is typically transmitted along an optical fiber.
In all fiber optic cables, the cumulative and combined effects of absorption and scattering attenuate the transmitted signals. Information is usually transmitted through fiber optic transmission lines using laser wavelengths of 1530-1565 nmxe2x80x94the so called C-Band wavelength region, and 1570-1620 nmxe2x80x94the so called L-Band wavelength region, where there are low attenuation loss windows. Although the signal attenuation rate in optical fibers is low within these bands, signal reduction with increasing transmission distance requires periodic signal amplification for long distance transmission.
In these systems, an Erbium-doped fiber amplifier (EDFA) is the most commonly used device to amplify all wavelengths simultaneously [xe2x80x9cErbium-doped Fiber Amplifiersxe2x80x9d, P. C. Becker, et. al., p. 335-346. Academic Press. 1999]. Due to the atomic properties of the Erbium ions in the silica fiber, the gain obtained by each of the wavelengths is different, and thus signals that enter with the same power into the amplifier can exit with power differences that can reach a few decibels [xe2x80x9cOptical Fiber Communication Systemsxe2x80x9d, L. Kozovsky, et. al., p. 578-584, Artech House, 1996]. Two spectral regimes of amplification within the C-Band wavelength region are known: A) 1530-1540 nm, and B) 1540 nm-1565 nm. The wavelength dependent amplification behavior in regime A is relatively linear, while the amplification in regime B is non-linear, due to the non-uniform spectroscopic behavior of the Erbium ion. The simultaneous existence of linear and non-linear gain regimes in the EDFA render the task of gain equalization over a wide range of gains extremely difficult. An amplifier that is not gain-equalized can have several serious consequences for the communication system, including a non-optimized power budget for a system operating in the entire C-Band Spectral range. Furthermore, in the case of a chain of a few amplifiers, each amplifier can enhance the power non-uniformly, and, in extreme cases, the wavelengths with smallest gain may be undetectable. Other serious consequences include cross-talk that can occur after traversing optical filters in components such as optical multiplexers or optical demultiplexers, and a non-optimized use of the population inversion of the Erbium-doped fiber amplifier because of strong amplified spontaneous emission radiation, or noise, in the strongly amplified wavelengths.
For the reasons mentioned above, a need for a gain equalization filter was identified by A. R. Charplevy (Charplevy et al., U.S. Pat. No. 5,225,922). Gain equalization is usually applied for a limited gain range (1-2 dB) across the full C-Band and is accomplished by means of passive filters with devices such as thin-film filters [xe2x80x9cDWDM Technologyxe2x80x9d, S. V. Kartalopoulos, p. 75-77, SPIE Press, 2000], Bragg gratings [Kartalopoulos, ibid. p. 78], long period gratings, and tapered fibers [xe2x80x9cOptical Networksxe2x80x9d, R. Ramaswami, et al., p. 101-102, Academic Press, 1998].
In dual-stage amplifiers there are other ways to achieve gain equalization, including choosing different types of Erbium fiber for the two stages (Sugaya et al., U.S. Pat. No. 6,055,092), different lengths of Erbium fiber (Alexander, U.S. Pat. No. 5,696,615), or by inserting devices such as filters, isolators or even attenuators between the two stages (Taylor, et al., U.S. Pat. No. 6,061,171; Alexander, U.S. Pat. No. 5,696,615). Dynamic filters, using acoustically tuned optical amplifiers, have also been suggested (xe2x80x9cFiber Based Acousto-optic Filtersxe2x80x9d, B. Y. Kim, et al., OFC 99, TuN 4, p. 199-201, Olshansky, U.S. Pat. No. 5,276,543).
In all the cases mentioned above, the gain equalization filter is usually suitable for a predetermined amplifier gain. When there is a need to change the gain, the power equalization of the different wavelengths will degrade and no longer be optimal.
Another conventional way to achieve dynamic gain equalization is by using two amplifier stages with opposite gain tilts [Yadlowsky M. J., U.S. Pat. No. 6,215,581]. Opposite tilt signs are achieved by differentiating the optical pump level for each of the stages. However, it is well known that in this case the dynamic gain equalization range, within a specific flattening tolerance, is limited.
As the equality of the power levels is important, in many applications an attenuator is inserted in front of the amplifier (Sugaya, U.S. Pat. No. 5,812,710) or between the amplifier""s stages (Taylor, U.S. Pat. No. 6,049,413), to lower the signal power and accommodate the need for optimized gain for power equalization. However, it is well known to those skilled in the art that this technique wastes energy, and degrades the amplifier""s signal to noise characteristics.
Recently, commercial devices for dynamic gain equalization have been developed. These devices utilize dynamic filters based on acousto-optic filters (Pearson, U.S. Pat. No. 5,514,413), liquid crystal filters (Kuang-Yi Wu, U.S. Pat. No. 5,963,291) and Mach-Zehnder filters (Miller, U.S. Pat. No. 5,351,325, Ranalli et. al., xe2x80x9cPlanar tapped delay line based, actively configurable gain-flattening filterxe2x80x9d, ECOC 2000, Vol. 3, p. 21). These devices have been employed in conjunction with optical amplifiers. However, they are cumbersome, suffer high insertion loss, require high power resources, are not xe2x80x9cstand-alonexe2x80x9d, and necessitate wavelength monitoring of the amplifier output in order to reach gain equalization.
Active gain tilting elements are presently employed for compensating for the linear gain regime B (1540-1564 nm) in EDFAs. Commercial companies such as Chorum and Sumitomo manufacture such components. However, it is important to emphasize that with these elements, high dynamic gain equalization can be achieved only in this linear spectral gain regime. Moreover, these elements require wavelength monitoring means, as well as complicated performance control means.
The use of a Thulium-doped fiber (Tm-fiber) as a passive gain tilting element at the output or inside an optical amplifier operating in the linear gain regime (1540-1564), was suggested by Kitabayashi et. al (U.S. patent application Ser. No. 20010017728A1, xe2x80x9cActive gain-tilt compensation of EDFA using Thulium doped fiber as saturable absorberxe2x80x9d, and ECOC 2000, Vol. 2, p.177). The Tm-fiber""s linear absorption characteristics, described by S. D. Jackson et. al. in xe2x80x9cTheoretical modeling of Tm-doped silica fiber lasersxe2x80x9d, J. of Lightwave Technology, Vol. 17, no. 5, p.948 (1999), are exploited for compensating for the Erbium fiber""s linear wavelength-gain dependence between 1540-1564 nm. The gain tilt of the Er-doped fiber and the loss tilt of the Tm-fiber are both linear with respect to the wavelength at 1540-1564 nm. The slopes of these linear tilts have opposite signs and their absolute values are similar (Kitabayashi et. al, above). However, the Tm-fiber compensates for the EDFA gain tilt only in the linear gain regime between 1540 nm and 1564 nm, where the Erbium wavelength-gain dependence is linear. Thus, this method is not suitable for compensating for the non-linear Erbium fiber regime in the 1530-1540 nm spectral region. Since present day communication systems operate in the full C Band (1530-1565 nm), any partial solution for gain equalization (for examplexe2x80x94gain flattening of only part of the spectrum) render such Tm-fiber/EDFA system practically useless.
A method utilizing excited state trapping in Erbium-doped Fluoride fiber has also been reported (M. J. Yadlowsky xe2x80x9cIndependent control of EDFA gain shape and magnitude using excited state trappingxe2x80x9d. J. of Lightwave Technology, Vol. 11, no. 5, p. 539 (1999)). However, it is well known to those skilled in the art that Fluoride fibers are not preferred in communication systems, due to their short degradation time and splice problems. In summary, there is at present no known low cost and simple solution for a dynamic gain equalized EDFA useful for the entire C-Band.
There is thus a widely recognized need for, and it would be highly advantageous to have, a dynamic gain equalizer for an EDFA based on fiber technology. The EDFA spectral shape should change dynamically, to simultaneously equalize the gain of all employed WDM channels in the entire C-Band, namely between 1530 nm-1565 nm, with a wide dynamic gain range.
The present invention is of a dynamic gain equalizer for a rare-earth doped amplifier, specifically an Erbium-doped Fiber Amplifier (EDFA). The dynamic gain equalizer is preferably based on the combined use of dual-stage amplifier basic bloc comprised of a Tm-fiber optically connected between two Erbium-doped fiber amplifier stages, constructed from identical or (spectrally, lengthwise, etc.) different Erbium-doped fiber types, and the use of active control of the elements of this basic bloc. In the following, we will refer generically to the two stages as xe2x80x9cfirst stagexe2x80x9d and xe2x80x9csecond stagexe2x80x9d, with the understanding that if the amplifier has more than two stages, the basic bloc used to explain the invention herein is repeated. Also, the order of the stages is not meant to be limiting, and it is possible to change it, so that the terms xe2x80x9cfirstxe2x80x9d and xe2x80x9csecondxe2x80x9d may be interchanged. The relevant aspect is that an output signal power from the xe2x80x9cfirstxe2x80x9d stage induces the saturation in a self-saturating absorber, specifically a Tm-fiber connected to this first stage.
The present invention provides a way of dynamically equalizing the gain across both the linear and non-linear regimes of the C-Band simultaneously. That is, the spectral response will be maintained relatively flat when the gain of the EDFA is changed, when the input power to the EDFA is changed while the gain is kept constant, or when the gain and input power are changed simultaneously. The present invention reaches these goals by preferably utilizing Erbium and Thulium-doped fibers together with extensive software algorithms, which control the pump power delivered to different portions of the Erbium doped fiber.
The present invention is also related to the extension of the wavelength-dependent gain range values, in which a dual (or multiple) stage EDFA output is flattened, within a specific tolerance.
The mechanism proposed is based on the tandem characteristics of an Erbium doped fiber and a Tm-fiber in a dual or multiple-stage EDFA. The Tm-fiber is employed in this embodiment as a self-saturable absorber. When low power is inserted into the Tm-fiber, the fiber functions as an optical filter with a large slope of linear wavelength-dependent loss, and with a certain level of excess loss (FIG. 1). The higher the power entering the Tm-doped fiber, the lower is the slope of the wavelength-dependent loss and the smaller is the excess loss level (FIG. 1). The saturation characteristics of a Tm-doped Silica fiber between 1530 nm-1605 nm is described by S. D. Jackson et. al. in xe2x80x9cTheoretical modeling of Tm-doped silica fiber lasersxe2x80x9d, J. of Lightwave Technology, Vol. 17, no. 5, p.948 (1999). However, as mentioned above, since the spectral gain profile of the EDFA is linear only between 1540-1565 nm. the mutual saturated absorption/emission characteristics of the EDFA""s Erbium doped and Thulium-doped fibers are absolutely required for achieving high dynamic gain equalization within the whole C-Band range.
When constructing a dynamic gain EDFA, which is gain-flattened at the full C-Band range for a wide range of gains (typically 6-12 dB), the wavelength dependent saturated gain and absorption of the combination of Erbium and Thulium fibers has to be carefully considered. Generally, the Erbium fiber section can perform dynamic gain equalization mainly in the non-linear regime of the EDFA (namely 1530-1540), while the combined saturation characteristics of Thulium and Erbium fiber perform the gain equalization in the linear regime (namely 1540-1565 nm). However, only a synergistic combination of three factors: a) the lengths of the Erbium and Tm doped fibers comprising each of the EDFA stages, b) the spectral characteristics of each Erbium doped fiber controlled by its pump level, and c) the wavelength dependent absorption of the Thulium, can perform dynamic gain equalization over the entire C Band.
In the high gain range (highly pumped first stage, thus high power signals entering the Tm-doped fiber and the amplifier second stage) the dual-stage amplifier gain tilt is reduced by the high power output signals exiting the first stage and saturating the mid-stage Tm-fiber and the weakly pumped second-stage Erbium fiber. The combination of the saturated Tm-fiber and the weakly pumped second stage Erbium fiber flattens the gain spectral profile in both the linear regime and the non-linear regime of the C-Band. Alternatively, in the low gain range (weakly pumped first stage, thus low power signals entering the Tm-fiber and the second stage), in which the gain tilt is negative, the first stage low power output signals experience a combination of small signal absorption in the Tm-fiber and of high spectral-dependent amplification in the second stage. This combination again flattens the gain spectral profile in both the linear and the non-linear regimes of the C-Band.
The main novel and inventive aspects of the method and apparatus of the present invention, in comparison with that of Kitabayashi et. al. (U.S. patent application Ser. No. 20010017728A1, and in ECOC 2000, Vol. 2, p.177), are summarized as follows: the present invention uses the self saturation of the Tm-fiber in a dual stage EDFA, which is induced by first stage output signals, combined with complementary gain spectral profiles of the two (or more) EDFA stages, which are achieved by controlling the pump level of each stage utilizing software algorithms, to yield a wide dynamic gain flattening over the entire C-Band. In contrast, in Kitabayashi""s dual-stage EDFA, the Tm saturation level is controlled by an auxiliary diode externally (i.e. it is not self induced by the power exiting the prior EDFA stage). Kitabayashi""s apparatus and method are essentially and ultimately based on the approximately linear and opposite tilts of the (externally controlled or pumped) Erbium and Tm-doped fibers, which hold only in the linear range (1540-1564 nm) of the C-Band. Thus, Kitabayashi""s method cannot, in principle, achieve gain flattening over the entire C-Band.
According to the present invention there is provided a method for dynamic gain equalization in an optical fiber system, comprising: a) providing a plurality of rare-earth doped amplifier stages; b) providing a self-saturable absorber optically connected between each two of the plurality of rare-earth amplifier stages, each of the two stages including a first and a second stage; and c) synergistically controlling the pumping of the first and second stages and the saturation of the self-saturable absorber connected therebetween, whereby the combined action of the pumping and the saturation provides gain equalization over a wide spectral range covering the entire C-band.
According to the present invention there is provided an apparatus for dynamic gain equalization over the entire C-Band in an optical fiber system, comprising: a) a plurality of rare-earth doped amplifier stages, each two of the plurality of stages including a first and a second stage; b) a self-saturable absorber optically connected between the first and second amplifier stages, the self-saturable absorber having a saturation property controlled by an output signal of at least one of the first and second stages; and c) control means for controlling an optical property of each of the first and second stages, the control means facilitating a synergistic action of the first and second stages and the self-saturable absorber, whereby the synergistic action provides optical gain equalization over a wide spectral range covering the entire C-band.
According to the present invention there is provided a method for dynamically equalizing the optical gain across both the linear and non-linear regimes of the C-Band, comprising: a) providing at least one dual-stage Erbium doped fiber amplifier having a first stage and a second stage; b) optically connecting a Thulium-doped fiber between the first and second stages; c) synergistically controlling the spectral output of the first and the second stages; and d) using the spectral output of the first stage to affect an optical characteristic of the Thulium-doped fiber, the affected optical characteristic combining with the first and second spectral outputs to induce a substantially equalized gain across the C-Band.
The apparatus of the present invention can be used as a stand-alone device or in conjunction with a dichroic filter, or any other passive gain equalizing filter, and can be an important contributor for dynamic gain equalization in Erbium-doped fiber amplifiers.